The present invention relates to devices and methods that use electroencephalographic responses to auditory stimuli to evaluate the hearing of a subject, and that are capable of weighting the response to an auditory stimulus based upon an analysis with normative data.
In the past, hearing impairment in babies and children was often not detected until after it was observed that the baby or child did not respond normally to sound. Unfortunately, it often took months or even years for the parent to observe the impairment, and by that time the child""s language and learning abilities were negatively and often irreversibly impacted. Indeed, recent studies indicate that the vocabulary skills of hearing impaired children markedly increase if their hearing loss is detected early. The optimal time to evaluate hearing loss is thus immediately after birth, both because early detection allows for early treatment, and because parents often fail to bring their infants to later appointments. As a result, a number of states have implemented programs to evaluate newborns for hearing loss.
However, babies, especially newborns, cannot participate in traditional hearing tests, which require the subject to indicate if he or she hears the auditory stimulus. Thus, devices and methods have been developed to objectively determine hearing loss, without the voluntary participation of the subject. One such method involves analyzing the involuntary electroencephalographic (EEG) signals evoked from a subject in response to an auditory stimulus. When a subject perceives a sound having particular characteristics, a specific EEG waveform known as an Auditory Brainstem Response (ABR) is generated. This ABR response signal is typically small in magnitude in relation to general EEG activity. Therefore, statistical and signal processing techniques have been employed and developed to help detect, to a pre-defined level of statistical confidence, whether an ABR response has in fact been evoked. ABR testing is especially applicable to evaluation of infants, but can be applied to any subject.
The ABR that is evoked in response to the auditory stimulus may be measured by use of surface electrodes on the scalp or neck. As a practical matter, the electrodes will also detect noise signals from neural activity (besides the ABR), muscle activity, and non-physiological environmental noises.
Typically, ABR testing becomes more difficult and time consuming with higher levels of noise, because noise obscures the evoked auditory response. Therefore, when conducting ABR testing, it is important to measure noise, and to control for the effects of such noise. The present invention provides for an improved method to optimize ABR hearing testing to account for the effects of noise. The improved method, which uses normative data, can shorten testing time and improve accuracy, thereby making such tests more attractive to parents and health care providers.
Several techniques have been used to minimize the physiological noise in the EEG response from an auditory stimulus (see M. Don and C. Elberling, Evaluating Residual Background Noise in Human Auditory Brainxe2x80x94Stem Responses, J. Acoust. Soc. Am. 96 (5), Pt. 1: 2746-2757 (1994)), including signal averaging and weighted signal averaging, signal filtering, artifact rejection, stimulus modification, targeted electrode placement, and various techniques designed to relax or sedate the subject.
The prior art details weighting techniques that evaluate the noise content in an EEG response and apply weights relative to the noise content of other EEG responses (see C. Elberling and O. Wahlgreen, Estimation of Auditory Brainstem Response, ABR, By Means Of Bayesian Inference, Scand. Audiol., 14:89-96, (1985); M. Hoke, B. Ross, R. Wickesberg, and B. Lutkenhoner, Weighted averagingxe2x80x94Theory and application to electric response audiometry, Electroencephalogr. Clin. Neurophysiol., 57, 484-489 (1984); B. Lutkenhoner, M. Hoke, and C. Pantev, Possibilities and limitations of weighted averaging, Biol. Cybernet., 52, 409-416, (1985)). However, this prior art does not provide for weighting that is anchored to an absolute minimum expected ABR amplitude based on normative data. In addition, prior art does not describe weighting the EEG response relative to one or more characteristics of ambient acoustic noise at the time of the auditory stimulus. Additionally, the prior art does not reveal or suggest the use of noise weighting in conjunction with signal analysis that transforms the EEG response into a series of polarities to determine the probability of the presence of an ABR response.
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The present invention provides a device and method for use in analyzing the EEG signal evoked in response to the auditory click stimulus, to determine if the subject suffers from hearing loss. Broadly, the invention is directed to devices and methods that are capable of weighting the response to an auditory stimulus based upon an analysis with normative data.
In one embodiment of the invention, evoked EEG responses to auditory stimuli are collected, and organized into xe2x80x9csweeps,xe2x80x9d with each sweep containing the response signal for one auditory stimulus. The sweeps are organized into C blocks, with each block c containing a number of sweeps Nc. The noise content of each block c is determined and compared with the allowable noise content associated with the absolute minimum expected ABR amplitude, from which an associated weight wc is calculated.
The response signal for each sweep is digitized and converted into a series of binary numbers (0 or 1) corresponding to whether the amplitude of the response signal is positive or negative at various points in time. The digitized, binary waveform is compared to a benchmark ABR waveform to determine if the ABR is present. To make this determination, a polarity sum is calculated, which represents the sum of the polarities of the response signals within a block at each measured point in time. The polarity sum of each block is multiplied with the associated weight, and combined into a polarity sum for all weighted blocks. Statistical techniques are then used to determine if an ABR is present, relying upon the expected distribution of polarity sums in the absence of an ABR. This expected distribution is developed theoretically. A xe2x80x9cPassxe2x80x9d is triggered if the observed polarity distribution, as represented in a specifically defined test statistic, indicates that the likelihood that an ABR is present is above a predetermined statistical threshold. After a certain number of blocks have been completed, evaluation will cease if a xe2x80x9cPassxe2x80x9d has yet not been triggered. Under such circumstances, the subject will be referred for further testing to determine if he or she in fact does suffer from hearing loss.
In accordance with the present invention, the polarity sum for each block is weighted based on its noise content in comparison with normative data for a hearing subject. These normative data reflect the maximum allowable noise content that would still permit testing for a hearing subject with a very weak ABR waveform. The present invention improves upon the prior art because it provides an accurate way to compensate for different or changing noise contents, therefore conserving evaluation times and resources, without sacrificing evaluation accuracy.
Although the embodiments described here are directed towards evaluation of newborn hearing, it is believed that the present invention can be applied to any evaluation, whether of hearing or not, in which evoked potentials are analyzed.
As described below, the present invention makes extensive use of normative data. These normative data were derived from analysis of clinical data, and from computer simulations representing different testing conditions. The normative data employed in the preferred embodiment of the present invention are reflected in the drawings described in the subsequent paragraph.